The field of the invention is motor control circuits, and particularly, control circuits which limit the amount of drive current to match the commutation limits of the motor.
The maximum current which a commutating motor can withstand varies as a function of speed. Control circuits for such motors include current limit circuits which prevent the current delivered by the motor control from exceeding the commutation limits of the motor. As shown in FIG. 3, the simplest of the prior control circuits limits the motor current at a fixed value regardless of motor speed, as shown by the straight line 1. More sophisticated motor controls provide current limiters which establish sloped current limit curves as shown by the tapered lines 2 and 3.
The actual commutation limits of a family of motors are usually published by the motor manufacturer, and these take the form of a "constant horsepower" curve which is hyperbolic in shape. While the multiple sloped commutation limit indicated by curve 3 in FIG. 1 may approximate such a constant horsepower curve, the establishment of such a curve involves the making of many adjustments to tailor the curve to each motor in the family. The more closely the multiple slope curve is made to approximate a constant horsepower curve the more adjustments which must be made.